1. Field of the Invention
This invention relates to electro-optic modulation systems. Still more particularly, this invention relates to low voltage laser modulators using two crystals.
2. Description of the Prior Art
The system of this invention was intially developed in the context of laser-based space communication systems. In such systems, it is necessary to obtain maximum performance, that is, high data rate, with minimal power consumption. Minimizing the power consumption of the system has several beneficial effects: power supply requirements are reduced; heat dissipation problems are reduced; and the reliability-lifetime of the system will be increased due to the lower switching voltages utilized.
Modulation systems based on the electro-optic polarization of light beams have been in existence for many years. Most of these systems are based on the Kerr effect in which electrically induced birefringence in either a liquid or a crystal, proportional to the applied field squared, causes a rotation of the plane of polarization of the light beam upon application of a suitable electric field. Devices which employ two capacitor plates inserted in a glass cell containing a suitable liquid are called Kerr cells. When a similar effect, proportional to the applied field, is induced in suitable crystals, the device is usually called a Pockels cell. The crystal or Pockels cell is the more commonly employed device. Such devices have been employed in a variety of different configurations to modulate light beams.
U.S. Pat. No. 3,956,626 to Ross discloses a "Pulse Quaternary Communication Means" directed to a high data rate laser communication system. Although bearing a superficial similarity in its structural organization to the instant invention, this prior art system in fact utilizes its various crystal modulators to perform four different operations on each laser pulse by manipulating both the polarizaton and the delay between pulses.
A more conventional use of the crystal modulators is found in U.S. Pat. No. 3,532,890 to Denton for "Optical Multiplexing And Demultiplexing Systems." This system multiplexes and demultiplexes by the use of a polarization switch which rotates the polarization of selected pulses 90.degree. for each shift and a polarization-selective coupler which combines the two separate orthogonally-directed pulse trains along a common beam direction. The polarization selective coupler also functions as a channel separator, thereby demultiplexing orthogonally polarized signals for propagation along two separate wave paths. This demonstrates a conventional use of the crystal modulators and, further, that the inputs to and the outputs from the individual crystal modulators do not bear a one-to-one correspondence with each other.
Other prior art of interest includes U.S. Pat. No. 3,638,024 to Chen, et al., for a "Optical Pulse Inverval Modulation System" and U.S. Pat. No. 4,145,607 to Bates for a "System And Method For Shaping Pulses Of Optical Radiation." None of these prior art systems disclose the system of the instant invention wherein crystals are utilized to transmit essentially all of the input light beam as essentially pure E component linearly polarized light, thus not requiring the additional voltage for modulation of the input light beam which is required when both E and O polarization components are modulated simultaneously.